<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

<script>

    var test = 20;
    test = "张三";
    alert(test);

    /*
        var:
            1.作用域：全局变量
            2.变量可以重复定义
    */



</script>
</body>
</html>